Diffusive-thermal Instability Research Articles

Three-dimensional diffusive-thermal instability of flames propagating in a plane Poiseuille flow

The three-dimensional diffusive-thermal stability of a two-dimensional flame propagating in a Poiseuille flow is examined. The study explores the effect of three non-dimensional parameters, namely the Lewis number Le, the Damköhler number Da, and the flow Peclet number Pe. Wide ranges of the Lewis number and the flow amplitude are covered, as well as conditions corresponding to small-scale narrow (Da≪1) to large-scale wide (Da≫1) channels. The instability experienced by the flame appears as a combination of the traditional diffusive-thermal instability of planar flames and the recently identified instability corresponding to a transition from symmetric to asymmetric flame. The instability regions are identified in the Le-Pe plane for selected values of Da by computing the eigenvalues of a linear stability problem. These are complemented by two- and three-dimensional time-dependent simulations describing the full evolution of unstable flames into the non-linear regime. In narrow channels, flames are found to be always symmetric about the mid-plane of the channel. Additionally, in these situations, shear flow-induced Taylor dispersion enhances the cellular instability in Le<1 mixtures and suppresses the oscillatory instability in Le>1 mixtures. In large-scale channels, however, both the cellular and the oscillatory instabilities are expected to persist. Here, the flame has a stronger propensity to become asymmetric when the mean flow opposes its propagation and when Le<1; if the mean flow facilitates the flame propagation, then the flame is likely to remain symmetric about the channel mid-plane. For Le>1, both symmetric and asymmetric flames are encountered and are accompanied by temporal oscillations.

Propagation characteristics of lean turbulent premixed ammonia–hydrogen flames

Direct numerical simulations (DNS) of fuel-lean turbulent premixed NH3-H2-N2-air flames are analyzed to investigate propagation and flame structural characteristics under fixed velocity and length ratios. To comprehensively assess the impact of diffusive-thermal imbalances on hydrogen–enriched ammonia flames, additional solutions with unity-Lewis-number transport were analyzed and compared with those obtained using the mixture-averaged transport model. The increase of H2 fraction in the fuel leads to elevated mean turbulent flame speed and stretch factor, indicating the impact of thermal-diffusive instability. The turbulent flame speed of the 60%NH3-25%H2-15%N2-air flame displays pronounced oscillations, a phenomenon absent in other mixtures considered in the current study. This behavior is attributed to the preferential diffusion of H2 mixed with the low-reactive NH3 in moderate quantities, resulting in higher generation of flame elements extending into the product side and dynamic evolution of H2. The flame structure analysis, in terms of conditional averages, revealed a distinctive variation in H2 and H atom distributions. The flames with a higher H2 fraction (40%NH3-45%H2-15%N2-air) produced a second peak of H2O2 in the trailing edge region, indicating additional production in the intense reaction zone. Additionally, in the 60%NH3-25%H2-15%N2-air flame, the reaction rate of H2 exhibited a unique behavior, with H2 being produced in the intermediate flame zone and rapidly consumed in the reaction zone, differing from other cases.

Flame propagation characteristics and thermal radiation hazards of methane-hydrogen-mixed cloud explosion in unconfined area: Experiment research and theoretical modeling

Under an equivalency ratio of 1.1, this manuscript studies the fireballs of methane-combustible gas clouds with varying hydrogen doping ratios (5 %, 10 %, 20 %, 30 %, 50 %, and 75 %). It then modifies the semiempirical-theoretical fireball heat radiation model based on flame size. Under 5%–30 % mixing ratio, flame growth characteristics are more comparable, and peak flame propagation speed is 18 m/s, whereas the mixing ratios of 50 % and 75 % are higher, take 0.1s to complete the accelerated process, and reach 26.7 and 36.7 m/s, respectively. In flames including methane hydrogen-doped mixtures, thermal diffusion instability and hydrodynamic instability are common. With an increase in the hydrogen doping ratio, the flame self-acceleration index shows a steady rising trend. Additionally, heat flux meter peak heat radiation values increase with hydrogen blending ratio, while the time to peak decreases. These findings align with the developmental process of flame morphology. Based on the observed flame development characteristics, a semi-empirical-theoretical model of heat radiation from the fireball is simplified. The results show that as the hydrogen doping ratio increases, the safety zone must be moved further away.

Confined deflagration of propanal/air in the presence of intrinsic instability at elevated pressures

The partial oxidation process is a chemical technique with a high risk of deflagration, which involves directly mixing of fuels with oxidizers at elevated conditions. Under non-standard conditions, the deflagration behaviors of fuels are often unavailable to the general public, which hampers progress in further basic research. In this study, the deflagration behaviors of propanal-air mixtures at 75 °C and 0.1–1.0 MPa were investigated, and the effect of initial pressure (P0) and equivalence ratio (φ) on the deflagration characteristics of propanal was discussed. To explain the similarities and differences in the deflagration characteristics under various conditions, the buoyancy instability, hydrodynamic instability and thermal-diffusion instability suffered by the flames were also analyzed. Results show that the pressure generated by the burned mixture is a cubic function of time, and the state of flame propagation can be determined by the monotonicity of the second-order derivative of pressure. Moreover, the peak deflagration pressure (Pm) is more sensitive to changes in φ on the fuel-lean side, but Pm/P0 at the fuel-rich condition is markedly higher than Pm/P0 at the fuel-lean condition. At all φ, an increase in P0 effectively shortens the ignition time and reduces the minimum ignition energy. Nevertheless, there are discernible differences in the variation patterns of the deflagration time (θm) with P0 at various φ. Specifically, θm gradually prolongs with increasing P0 at fuel-lean conditions, whereas it tends to be the same at different P0 and φ = 1. Furthermore, under fuel-rich conditions, θm progressively shortens as P0 increases. The decrease in θm is attributed to the increasing flame speeds in the presence of intrinsic instability, and the self-accelerating flame yields pressure waves, which travel faster than the flame. Then a series of reflected pressure waves are formed after the pressure waves strike the vessel wall, interacting with the flame front and resulting in the pressure oscillation. Moreover, increasing P0 advances the pressure oscillation and augments the intensity of pressure oscillation.

Turbulent Flame Speed and Flame Characteristics of Lean Premixed H2-CH4 Flames at Moderate Pressure Levels

Abstract Carbon dioxide emissions in gas turbine power generation can be reduced by adding an increasing amount of hydrogen to the existing natural gas-fueled combustion systems. To enable safe operation, more insight on how H2 addition affects turbulent flame speed and other important flame characteristics is needed. In this work, the investigation of hydrogen addition effects on certain flame properties has been carried out in a high-pressure axial-dump combustor at gas turbine relevant conditions. OH planar laser induced fluorescence (PLIF) was applied to retrieve flame front contours and turbulent flame speed. The results show that as the concentration of hydrogen in the fuel mixture increases, turbulent flame speed and flame characteristics change drastically. Two main regimes can be identified: From 0 to 50% vol. Hydrogen, the turbulent flame speed increases weakly in an almost linear fashion, while from 50% vol. to 100% vol. the trend sharply changes and the higher reactivity of hydrogen, combined with a lower Lewis number, cause thermal-diffusive instability and preferential diffusion effects to become increasingly strong, leading to very high burning rates. The presented results help to understand and to define the relevant modifications that are necessary to successfully operate gas turbine combustor systems with high H2 content fuels.

Propagation and extinction of premixed H2[sbnd]O2[sbnd]N2 edge-flames

The effects of equivalence ratio (ϕ), global strain rate (σ) and mixture strength on the shapes and propagation rates (Uedge) of premixed edge-flames in H2O2N2 mixtures were investigated experimentally using a counterflow slot-jet apparatus. Results are presented for both single (premixed gas against cold inert gas) and twin (premixed gas against premixed gas) configurations. Both advancing (Uedge > 0) and retreating edge-flames (Uedge < 0) were characterized. Flame images show that for lean mixtures where H2 is the deficient species and thus the effective Lewis number (Leeff) is less than unity, the leading edge-flame has a stronger burning intensity than the trailing strained premixed flame. For mixtures with low ϕ (thus low Leeff) at near-extinction conditions corresponding to sufficiently high or low σ, transitions from continuous to broken structures were observed due to diffusive-thermal instabilities. Such transitions enable flames to survive under conditions where extinction would have occurred without transition. The combined effects of ϕ, adiabatic flame temperature and σ were found to correlate well in terms of the effect of a non-dimensional strain rate (ε) on a scaled propagation rate Ũ.

On accelerative propagation of premixed hydrogen/air laminar and turbulent expanding flames

The accelerative propagation of laminar and turbulent premixed hydrogen/air flames are investigated using a constant volume combustion chamber. The relative flame accelerative characteristics are analyzed under different φ, P, and u’. The results show that three distinct stages, namely quasi steady, transition acceleration and saturation acceleration stages are distinguished in laminar flame propagation process, and the onsets of saturation acceleration and transition acceleration nearly satisfy the relationship of Pecr2/Pecr1≈2. The temporal acceleration exponents (αR) of laminar flames increase quickly in transition acceleration stage and nearly remain as a constant (1.1–1.25) in saturation acceleration stage. The acceleration exponents (α) of laminar flames are weakly dependent on flame conditions once the cellular structures are settled. The turbulent flame propagation follows a continuous acceleration law as SF/SL,b∼Pedt and no evident transition point is observed. The temporal acceleration exponents of turbulent flames continuously increase as flame propagates outwardly and are susceptible to turbulence intensity. The acceleration parameters (dt) of turbulent flames continuously increase with u’/SL in different flame regimes and the growth trend becomes slow in corrugated flamelets regime. Besides, the dt of turbulent flames are promoted in the fuel-lean conditions. All of these indicate that the accelerative propagations of turbulent H2/air flames are dominated by turbulent stretch and the diffusive-thermal instability still plays a promotion role.

Experimental and numerical studies on characteristics of LPG-air premixed flames in curved tube

Liquefied Petroleum Gas (LPG) is the widely used fuel worldwide from residential to industrial applications, and thus largely meets the energy demand of various systems. The present study experimentally and numerically investigates the flame propagation behavior of LPG- air mixture in straight and curved tube with inner diameter 30 mm, a length of 500 mm for straight tube and a chord length of 500 mm for the curved tube. Numerical study has been carried out using ANSYS Fluent with FGM approach for modelling of combustion. The premixed reactant in the equivalence range of 0.3 ≤ φ ≤ 2.7 is supplied through the closed end of the tube and the mixture is ignited at the open end exposed to atmosphere. Different flame regimes are observed for 30 mm diameter combustor tube, like the self- repetitive extinction and re-ignition flame, forced repetitive extinction and re-ignition flames, cellular flames, blow out flames, stretched or slanted flames. The curvature of the tube was found to enhance the instabilities occurring in the flame, but rapid changing of flame speed towards the cold inlet is not observed in curved tubes. Cellular flames were formed in the curved combustor as a result of secondary flow and thermal-diffusive instability. Curvature in the combustor tubes induces cross stream vortices in the flow due to the secondary flow, which influence the flame structure in the fuel rich regime of the mixture.

Diffusive-thermal instabilities of a planar premixed flame aligned with a shear flow

The stability of a thick planar premixed flame, propagating steadily in a direction transverse to that of unidirectional shear flow, is studied. A linear stability analysis is carried out in the asymptotic limit of infinitely large activation energy, yielding a dispersion relation. The relation characterises the coupling between Taylor dispersion (or shear-enhanced diffusion) and the flame thermo-diffusive instabilities, in terms of two main parameters, namely, the reactant Lewis number L e and the flow Peclet number P e . The implications of the dispersion relation are discussed and various flame instabilities are identified and classified in the L e - P e plane. An important original finding is the demonstration that for values of the Peclet number exceeding a critical value, the classical cellular instability, commonly found for L e < 1 , exists now for L e > 1 but is absent when L e < 1 . In fact, the cellular instability identified for L e > 1 is shown to occur either through a finite-wavelength stationary bifurcation (also known as type-I s ) or through a longwave stationary bifurcation (also known as type-II s ). The latter type-II s bifurcation leads in the weakly nonlinear regime to a Kuramoto-Sivashinsky equation, which is determined. As for the oscillatory instability, usually encountered in the absence of Taylor dispersion in L e > 1 mixtures, it is found to be absent if the Peclet number is large enough. The stability findings, which follow from the dispersion relation derived analytically, are complemented and examined numerically for a finite value of the Zeldovich number. The numerical study involves both computations of the eigenvalues of a linear stability boundary-value problem and numerical simulations of the time-dependent governing partial differential equations. The computations are found to be in good qualitative agreement with the analytical predictions.

Stability of diffusion flames under shear flow: Taylor dispersion and the formation of flame streets

Diffusion flame streets, observed in non-premixed micro-combustion devices, align parallel to a shear flow. They are observed to occur in mixtures with high Lewis number (Le) fuels, provided that the flow Reynolds number, or the Peclet number Pe, exceeds a critical value. The underlying mechanisms behind these observations have not yet been fully understood. In the present paper, we identify the coupling between diffusive-thermal instabilities and Taylor dispersion as a mechanism which is able to explain the experimental observations above. The explanation is largely based on the fact that Taylor dispersion enhances all diffusion processes in the flow direction, leading effectively to anisotropic diffusion with an effective (flow-dependent) Lewis number in the flow direction which is proportional to 1/Le for Pe≫1. Validation of the identified mechanism is demonstrated within a simple model by investigating the stability of a planar diffusion flame established parallel to a plane Poiseuille flow in a narrow channel. A linear stability analysis, leading to an eigenvalue problem solved numerically, shows that cellular (or finite wavelength) instabilities emerge for high Lewis number fuels when the Peclet number exceeds a critical value. Furthermore, for Peclet numbers below this critical value, longwave instabilities with or without time oscillations are obtained. Stability regime diagrams are presented for illustrative cases in a Le−Pe plane where various instability domains are identified. Finally, the linear analysis is supported and complemented by time dependent numerical simulations, describing the evolution of unstable diffusion flames. The simulations demonstrate the existence of stable cellular structures and show that the longwave instabilities are conducive to flame extinction.

Determining the global activation energy of methane–air premixed flames

In this work, the determination of the apparent activation energy of a global chemical reaction mechanism of the methane–air flames is revisited. The one-step formulation allows to derive the theoretical background for the method to measure the activation energy within the burner stabilised flame setup. The validity of this approach is demonstrated by using the numerical simulations with the detailed reaction model and direct thin filament pyrometry measurements of the temperature distribution in flame. The combination of numerical and experimental approaches allows us to find the activation energy for various mixture compositions. The prediction of numerical simulations and measured values of the activation energy is found to be in good agreement with each other and the data known from the literature. It is demonstrated that two critical phenomena need to be taken into account to obtain a reliable estimate of the activation energy: the flame blow-off and the onset of the diffusive-thermal instabilities. The effect of these critical events on the accuracy of the measurements is discussed as well as prospects of further investigation.

Effect of initial pressure on the critical characteristics and overpressure of hydrogen-air premixed gas combustion and explosion

In order to ensure the safe utilization of hydrogen, the flame characteristics and explosion overpressure at lean-burn, stoichiometric and rich-burn conditions are investigated systematically by varying initial pressures. Experiments were conducted in a constant-volume combustion chamber using the Schlieren system to investigate the critical characteristics of hydrogen-air premixed flames at different equivalence ratios (from 0.2 to 6.5) and initial pressures (from 50 kPa to 150 kPa). The cellular instabilities of hydrogen-air premixed flames were interpreted and evaluated from the viewpoint of thermal-diffusive and hydrodynamic instabilities. The influence of initial pressure on the cellular instability and combustion and explosion characteristics were analyzed. The results show that the variation of thermal-diffusive instability dominantly governs the cellular instability at lean-burn conditions (φ = 0.3–1.0). In contrast, the cellular instability at rich-burn conditions (φ > 1.5) is governed by the variation of hydrodynamic instability. The increase of initial pressure promotes the cellular instabilities noticeably caused by the rise of the hydrodynamic instability. With the increase of P0, the Lewis number Le and thermal expansion ratio σ almost remain the same while flame thickness δ noticeably decreases. Increasing initial pressure at lean-burn and stoichiometric conditions (φ < 2.0) facilitates cellular instability, making the explosion more intense and resulting in higher explosion overpressure. In contrast, the variation of initial pressure suppresses the laminar combustion at rich-burn conditions (φ > 3.0).

Onset of flame acceleration in propane–oxygen mixtures

In this study, the onset of flame acceleration because of Darrieus–Landau and diffusive–thermal instability in propane–oxygen mixtures was investigated using experiments and analysis. Using the soap bubble method, the unstretched laminar burning velocity Su0 and critical flame radius rc at which flame acceleration occurs of the propane-oxygen mixture were evaluated. The experimental values of Su0 and critical Péclet number Pec agreed qualitatively with the calculated values, and Pec tended to decrease with increasing equivalence ratios. In addition, the dependency of Pec on the Markstein number Mab and the critical Karlovitz number Kac on Mab for the propane–oxygen flame was compared with that for other fuel–air mixtures. The comparison revealed that the decreasing trend of Kac with Mab for the propane-oxygen flame was qualitative consistent with other fuel-air mixtures, in contrast to the different slopes of the Pec-Mab relationship.

Experimental study of laminar and turbulent spherical flame propagation characteristics of hydrogen-rich syngas

Hydrogen-rich syngas, produced from biomass-based feedstocks, is a promising alternative energy source. However, its combustion characteristics are completely different from those of typical hydrocarbon fuels due to the presence of hydrogen. As a result, the kinetic mechanism of hydrogen-rich syngas premixed flame (HRSPF) propagation is very complex. In this work, the propagation of HRSPF was investigated employing different fan speeds (0–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0 bar–3.0 bar), and equivalence ratios (0.6–1.0). It was found that the propagation speed of turbulent HRSPF increased when the mixing process of turbulent flow was enhanced or the stretching effect of flame front was weakened. When the turbulence intensity increased, the effect of flame inherent instability on the propagation speed of HRSPF was weakened. When the relative changes of thermal diffusion and mass diffusion of flame front were considerable, the effect of diffusional-thermal instability on flame acceleration process was more significant. On the contrary, the effect of hydrodynamic instability on flame acceleration process was more significant.

Cellularity and self-similarity of hydrogen expanding spherical flames at high pressures

The onset of transition to cellularity and self-similar propagation of centrally ignited, expanding spherical flames in a reactive environment of H2/O2/N2 and H2/O2/He mixtures at initial pressures up to 15 bar were experimentally investigated using a newly developed, constant-pressure, dual-chamber vessel and were theoretically interpreted based on linear stability theory. The experiments were well-controlled to identify the separate and coupled effects of Darrieus–Landau instability and diffusional–thermal instability. Results show that the critical radius, rcr, for the onset of cellular instability varies non-monotonously with initial pressure for fuel-lean and stoichiometric H2/O2/N2 flames. This non-monotonous pressure dependence of rcr is well captured by linear stability theory for stoichiometric flames. The experimental critical Peclet number, Pecr = rcr/δf, increases non-linearly with the Markstein number, Ma, which measures the intensity of diffusional–thermal instability. However, a linear dependence of Pecr on Ma is predicted by linear stability theory. Specifically, the theory shows well quantitative agreement with the experimental results for mixtures with near-unity Leeff; however, it under-predicts the Pecr for mixtures with off-unity Leeff. In addition, there exists three distinct propagation stages for flames subjected to cellular instability, namely, smooth expansion, transition propagation, and self-similar propagation. The acceleration exponent, α, in the self-similar propagation stage was extracted based on the power-law of drf/dt = αA1/αrf(1 − 1/α), where rf is the instantaneous mean flame radius, and A is a constant. The values of α are located between 1.22 and 1.40, which are smaller than the suggested value (1.5) for self-turbulization.

Linear Stability of the Activation-Energy-Asymptotics Reactive-Layer Structure: I. Liñán’s Premixed-Flame Regime

ABSTRACT Overall linear instability characteristics of an asymptotically thin reactive layer in a flame are investigated, particularly for Liñán’s premixed-flame regime of activation-energy asymptotics. The reactive-layer instability analysis, formulated with the characteristic length and timescales of the inner reaction zone, is carried out not only by a numerical method but also by an asymptotic method, with the small expansion parameter obeying the distinguished limit of and , where , , and denote the downstream heat-loss factor, the Zel’dovich number, and the Lewis number, respectively. The asymptotic analysis yields an explicit dispersion relation, which is comprised of three distinct destabilizing or stabilizing contributions: (i) the reaction sensitivity associated with the downstream heat-loss effect, (ii) the reaction sensitivity associated with the preferential diffusion effect, and (iii) the diffusive relaxation. Three instability modes, namely cellular instability, planar instability, and pulsating instability, are found to be similar to the conventional diffusional-thermal instability. Consequently, the reactive-layer instability is seen to be good enough to serve as a general baseline for diffusional-thermal instability, despite lacking the detailed instability characterization in the proximity of , the corrections for which can be achieved by the conventional diffusional-thermal instability analysis within the NEF (nearly-equidiffusional flame) limit.

Experimental investigation of cell generation in an expanding spherical hydrogen-air flame front

This paper reports on the cellular structure formation on the front of a spherically expanding hydrogen-air flame. The hydrogen-air mixture was considered with hydrogen concentration in experiments from 10 to 50 vol%. This paper aims to analyze cell cascade formation, which occurs due to diffusional-thermal and hydrodynamics instabilities. Using experimentally obtained schlieren images, the flame front radius as the function of an angle was obtained. The cell amplitude dependencies on the normalized time were also analyzed. The values corresponding to cell splitting were obtained by the discrete Fourier transform method. The cell split criterion, which allows taking into account the known instability mechanisms, was formulated.

Experimental and theoretical investigation of ethyl methyl carbonate/air flames: Laminar burning velocity and cellular instability at elevated pressures

Ethyl methyl carbonate (EMC) gained great attention because of its advantages shown in practical applications both as a blend with conventional oil-derived liquid fuels, and as a solvent being used in Li-ion cell electrolyte compositions. Thus, a better knowledge of the combustion characteristics of EMC/air premixed flames is necessary, which has not yet been well-explored. Towards this goal, the laminar flame propagation of EMC/air mixture was investigated using a constant-volume combustion chamber at the initial pressures of 1, 2, 5 and 8 atm, with the unburned temperature of 423 K and the equivalence ratios ranging from 0.7 to 1.5. A recently established chemical kinetic mechanism of EMC by Takahashi et al. (Combust. Flame 2022) was adopted for the theoretical prediction, which accurately reproduced the measured LBVs at 1–5 atm; however, it underpredicted the LBVs for the lean flame condition at 8 atm. According to the flame instability analysis, the increase in pressure leads to the elevated hydrodynamic instability and the decreased critical radius of spherical premixed flame. Meanwhile, the rich flames of EMC/air had the smaller critical Peclet numbers and suffered the more severe thermal-diffusive instabilities.

Experimental and kinetic study on laminar burning velocities of ammonia/ethylene/air premixed flames under high temperature and elevated pressure

The laminar burning velocities (LBVs) of ammonia (NH3) blended with ethylene (C2H4) were measured at different blending ratios (α = 0–1), initial temperatures (Tu = 300–440 K), and initial pressures (Pu = 1–5 atm). The flame morphology showed that the NH3/C2H4/air flames suffer from cellular instability under elevated pressures (Pu = 5 atm). The Lewis number (Le) was used to evaluate the thermal diffusion instability. Le was insensitive to the initial pressure and positively correlated with the blending ratio. The hydrodynamic instabilities were investigated under various initial conditions, which were exacerbated by elevated initial pressures and large blending ratios. The LBVs and hydrodynamic instabilities of NH3 were enhanced by adding C2H4. The existing kinetic models can only accurately predict measured data within a narrow range. Based on Konnov Mech (2021), a new detailed kinetic model with 119 species and 1,365 reactions was developed, which could satisfactorily predict the LBVs of NH3/C2H4/air flames under current conditions. The analysis of reaction flux, sensitivity, and radical concentration in the pure and blended fuels provided deep kinetic insights. The consumption pathways of NH3 and C2H4 were diversified after blending. NH2, an important precursor of ammonia oxidation, adds a consumption channel with HO2, whereas HCCO, an important intermediate of ethylene oxidation, adds new channels with NO. H + O2 = OH + O, a chain branching reaction, significantly contributes to the LBVs of NH3/C2H4/air flames. The peak concentrations of H, O, and OH radicals increased as the C2H4 percentages increased. The relationship between the H radical concentration and blending ratio was virtually the same as that for the LBVs.

Experimental characterization of diffusive-thermal instabilities in CO2-diluted H2[sbnd]CH4[sbnd]CO unstrained diffusion flames

The combustion of multi-fuel mixtures is experimentally studied for the first time in unstrained diffusion flames, where the parasitic hydrodynamic effects present in common research burners are negligible. A broad range of H2COCH4 fuels highly diluted in CO2 is investigated to provide an understanding of the intrinsic diffusive-thermal instabilities (DTIs) that onset in low calorific biomass-derived syngas. For each fuel blend, the burning intensity or the Damköhler number (Da) is gradually reduced, going through the marginal stability state where DTIs first appear, down to the lean extinction limit. Flame stability limits are provided. From the large difference between the Lewis numbers of the multiple fuel species (Lei), the cells that onset due to H2 are seen to interact and compete with the pulsations from CO and CH4, leading to superimposed cellular-pulsating instabilities. These are thoroughly characterized by measuring the pulsations amplitude, frequency, cell size, number of cells, and fraction of the flame sheet actively burning. An effective fuel Lewis number (LeF,eff) calculated from the fuel mixture composition is introduced and used along with the Damköhler number to map the DTIs observed. At lower LeF,eff and Da, the cellular attributes of the superimposed instabilities dominate, while at larger Lewis numbers and the near marginal stability state, pulsations prevail.